The present disclosure relates generally to process control for reducing air pollution emissions, and more particularly, to improving nitrogen oxide removal by optimizing control of a selective catalytic reduction (SCR) process.
SCR is a process that converts nitrogen oxides (NOx) via a catalyst to into diatomic nitrogen (N2) and water (H2O). Typically, ammonia (NH3) or aqueous ammonia NH4OH is injected and mixed with gases that pass through a catalyst grid to reduce NOx. In a conventional SCR control system, a controller measures inlet NOx, outlet NOx and ammonia flow in real time. Logic in the controller adjusts the ammonia injection rate via an ammonia control valve to minimize residual NOx emissions. Typically, real world inefficiencies result in some ammonia slip, which is unreacted NH3 downstream of the SCR catalyst. NOx removal requirements are dictated by hourly and daily limits. As attempts are made to drive outlet NOx closer to zero, more ammonia must be injected into the process, resulting in an increase in ammonia slip emissions. The operating objective is to attain the desired level of NOx reduction while minimizing ammonia slip, since excess ammonia slip from over-control is undesirable and wasteful from a cost perspective.
Ammonia injection control logic continuously monitors heat input, SCR NOx, stack NOx, stack O2 and ammonia flow, determines ammonia control valve action, and mixes the ammonia solution into a slipstream of stack gas for delivery to an ammonia injection grid (AIG). In current implementations, the setpoint is statistically set based upon past analysis to ensure compliance with the current level of performance. Actual emissions are not tracked automatically so any adjustments to setpoints for periods of high emissions are done manually by an operator. The volume of NOx to be controlled is derived from estimating the volume throughput from Combustion Turbine (CT) Megawatts and ambient temperature. Exhaust flow rate is multiplied by corrected SCR NOx measurements and adjustment is made for additional NOx from duct burner firing based on duct burner flow rates. The percent of NOx removal required is calculated by measuring the difference between the SCR NOx concentration and the setpoint and dividing the same by the SCR NOx concentration value. This is adjusted by an additional factor calculated by trimming logic that factors in the compliance NOx measurement and additional factors to determine if an adjustment is needed to maintain compliance.
Current ammonia dosing logic is reliant upon empirical relationships and assumptions, and therefore is not a true chemical mass balance. The determination of SCFH of ammonia required is calculated by multiplying the estimated volume of NOx by the percent removal calculation. A conversion factor of 1.35 is used which assumes that all NOx is NO2, since removal of NO2 requires more ammonia than NO. For the actual ammonia flow measurement, aqueous ammonia is monitored in real time in pounds per hour. The flow is converted to SCFH of ammonia. The controller compares the calculated ammonia requirement to the actual ammonia flow and adjusts the ammonia injection valve as necessary.